1. Field of the Invention
The present invention relates to the precise measurement of short distances by means of a radar and, notably., the measurement of a level of liquid in a vessel in the presence of parasitic reflexions.
2. Description of the Prior Art
A known method for measuring a level of liquid in a vessel by means of a radar consist in placing the radar above the vessel, vertically to the surface of the liquid, and in making it send out a sequence of non-modulated microwave sinusoidal signals s.sub.k whose discrete pulsations are evenly distributed in a frequency band B where the coefficient of reflexion of the surface of the liquid does not vary notably. EQU s.sub.k =u exp j(w.sub.0 +k.DELTA.w)t!
u being an amplitude coefficient, w.sub.0 being the initial angular frequency, .DELTA.w being the change in angular frequency at the passage from one signal to another, t being the time variable and k being a positive integer that varies between 0 and N-1, N being the number of elements of a sequence of signals.
The signals received by the radar coming from the reflexion of the signals sent out by the surface of the liquid and by different parasitic obstacles are demodulated by the signals set out and give rise, in baseband, to signals r.sub.k having the form: ##EQU1##
M being an integer representing the number of obstacles that send back echoes, whether it is by the major lobe or by the minor lobes of the radar, i being a positive integer that varies from 0 to M-1, .nu..sub.i being an amplitude coefficient depending on the coefficient of reflexion of the i.sup.th obstacle, and .tau..sub.i being the time taken by the transmitted signal to go from the radar to the i.sup.th obstacle and return therefrom, the obstacle having the index 0 being the surface of the liquid.
The set of signals {r.sub.k } carries out a sampling, in the field of the frequencies, of a function resulting from a summation of sinusoidal waves whose periodicity values depend on the .tau..sub.i values and hence on the distance d.sub.i from the different obstacles since: EQU .tau..sub.i =2d.sub.i /c
c being the velocity of propagation of the waves (3 10.sup.8 m/s).
To extract the values of the times .tau..sub.i and hence obtain knowledge of the distances from the different obstacles, the usual practice is to equip the reception part of the radar with a processing circuit that carries out a discrete and reverse Fourier transform on all the received and demodulated signals {r.sub.k } which brings about a passage from the frequency domain to the time domain.
The values of .tau..sub.i correspond to maximum values of the function of the time obtained, the greatest maximum value corresponding to .tau..sub.0 for the level of liquid is supposed to give the most powerful echo.
According to Woodward's formula, the precision G of the measurement of the period .tau..sub.0 giving the level in the vessel is a function of the scanned frequency band B and of the energy signal-to-noise ratio R: ##EQU2##
In this use, this is not strictly speaking a limitation for the energy signal-to-noise ratio R varies little with the distance since the illuminated surface of liquid increases with the distance owing to the aperture of the illumination cone of the radar beam and may be great owing to the shortness of the distances to be measured. Thus, in theory, it will easily be possible to arrive at a precision of about 1 centimeter with a scanned frequency band of the order of 1 GHz.
In fact, there is another limitation due to the weakness of resolution of a discrete Fourier transform. Indeed, an elementary filter at output of a discrete and reverse FFT has a response in the temporal domain that is not infinitely narrow but has a major lobe with a width at 3 dB equal to the reverse 1/B of the scanned frequency band surrounded by minor lobes so much so that it is not possible to separate two echoes, one of which is a useful echo and the other is a parasitic echo, when they return to the radar at instants separated by a period of less than 1/B. In the case of a scanned frequency band of 1 GHz, the period is 10.sup.-9 seconds which corresponds, for the transmitted wave, to a to-and-from propagation distance of 15 cm. The result thereof is a lack of precision in distance measurement that is far greater than what might be expected from the Woodward's formula in this specific case.
One approach that might be considered to overcome this drawback would be to replace the discrete and reverse Fourier transform by a high resolution method for the localization of radiating sources. This approach has the advantage of having an infinite power of asymptotique resolution that is solely a function of the observation time. This type of method is well known in the prior art and is described, for example, by Georges Bienvenu and Laurent Kopp in Methodes haute resolution pour la localisation de sources rayonnantes (High resolution methods for the localization of radiating sources) in L'onde electrique, July-August 1984, Vol. 6, No. 4, pp 28 to 37.
As shall be seen hereinafter, the implementation of a high resolution method for the localization of radiating sources requires, as a preliminary, the formulation of a hypothesis on the maximum number of useful and parasitic echoes liable to be encountered. This maximum number of echoes is then taken into account in the method which localizes them all, whether they are real or fictitious, in model-making attempt wherein the fictitious echoes may give rise to a response greater than that of the real echoes. There then arises a problem of identification of the useful echo from among the real and fictitious echoes localized by a high resolution method.